Cancer cells undergo dramatic changes in carbohydrate expression during the onset and progression of disease. As a result, there has been considerable interest in understanding why these changes occur, how they contribute to the disease, and exploiting these changes for clinical use. Carbohydrate binding antibodies and lectins play a critical role in this research for several reasons. First, antibodies and lectins are used extensively to detect and monitor changes in carbohydrate expression. In this capacity, they are used for basic research, as diagnostics, and as therapeutic agents. A comprehensive understanding of their specificity is critical for proper use and interpretation of results. Until recently, this information was not readily available. With the advent of carbohydrate microarray technology, we can now rapidly analyze binding of these proteins to a wide variety of carbohydrate antigens. In addition, the array can be used to develop new antibodies and lectins for monitoring new carbohydrate antigens. We have been developing antibodies to several carbohydrate antigens are using the antibodies to study carbohydrate expression in tumors. Second, many natural carbohydrate binding proteins such as galectins are involved in tumorigenesis and metastasis. As a result, there has been considerable interest in developing ligands to modulate their activity. Our carbohydrate microarray is well-suited to evaluate binding and identify ligands for natural lectins.